Many surge protection devices utilize varistors (also known as voltage dependent resistors or VDRs) as the main component for diverting electrical disturbances associated with industrial distribution equipment. A varistor is an electronic component with a “diode-like” nonlinear current-voltage characteristic. The basic function of a varistor is to conduct significantly increased current when voltage is excessive, while limiting residual voltage to protect circuits. Varistors are often used to protect circuits against excessive transient voltages by incorporating them into the circuit in such a way that, when triggered, they will shunt the current created by a high voltage away from sensitive components.
The most common type of varistor is a metal oxide varistor (MOV). An MOV contains a ceramic mass of zinc oxide grains in a matrix of other metal oxides (such as small amounts of bismuth, cobalt, manganese) sandwiched between two metal plates (the electrodes). The boundary between each grain and its neighbor forms a diode junction, which allows current to flow in only one direction. The mass of randomly oriented grains is electrically equivalent to a network of back-to-back diode pairs, each pair in parallel with many other pairs. When low voltage is applied across the electrodes, only a tiny current flows, caused by reverse leakage through the diode junctions. Accordingly, the MOV will look like an open circuit. When the voltage applied across the electrodes exceeds the MOVs “maximum continuous voltage” (MCOV) rating, the diode junction breaks down due to a combination of thermionic emission and electron tunneling, and a large current flows. The result of this behavior is a highly-nonlinear current-voltage characteristic in which the MOV acts as an insulator (high resistance) at low voltages and acts as a conductor (low resistance) for transients that exceed the MOV's maximum continuous voltage.
The maximum continuous voltage rating defines maximum AC or DC voltage that can be applied before the MOV begins to conduct. Once the maximum continuous voltage is exceeded the MOV begins to heat as conduction current flows. In this state, the MOV will eventually have a catastrophic failure due to thermal runaway. To address this problem, surge protection device (SPD) manufacturers have incorporated thermo-mechanical disconnection systems to disconnect the MOV from the supply voltage before it fails. However, there is a limitation to the use of thermo-mechanical disconnection systems that relates to the magnitude of excess voltage that can be applied. If the voltage is sufficiently high enough, the heating of the MOV occurs too quickly to allow the thermo-mechanical disconnection system to react. This is due to the rate of conduction current rise (di/dt). In this regard, the current cannot be dissipated through the MOV and “hot spots” or areas of concentrated current density rapidly form in the MOV, thereby causing a puncturing of the MOV.
Moreover, while MOVs offer low costs and relatively high transient energy absorption capability, MOVs progressively degrade with repetitive stress. Accordingly, an MOV will age quicker with more frequent activation. In a typical surge protection device, at least one MOV is employed per mode of protection and sometimes two or more MOVs are used in parallel to achieve higher surge current ratings. The MOV maximum continuous voltage rating is selected to provide maximum protection. However, the MOV maximum continuous voltage rating must be sufficiently high enough relative to the system voltage so that the MOV is only active during a surge event. If the MOV is activated when there is no surge event, then the MOV will age more quickly due to fatigue. One common design convention is to select an MOV having a maximum continuous voltage rating of 115% of nominal system voltage to avoid unwarranted activation.
The present invention provides a new and improved surge protection device that solves the problems described above and overcomes drawbacks of prior art solutions, such as use of gas discharge tubes (GDTs) or crowbar circuits comprising zener diodes and silicon-controlled rectifiers (SCRs) (also known as a thyristor).
The present invention provides a new solution to the problem described above, and provides advantages that are not found in existing prior art solutions.